ABSTRACT:

This paper reports on the design and constructional aspects of geotextile included granular mattress foundations of Khulna Medical College in Bangladesh. These foundations were envisaged, designed and constructed on soft soils for four and five storied buildings. Four buildings, including the Academic Building, Nurses Training Centre and two Hostel Buildings were constructed. The granular mattresses were constructed by laying a nonwoven needle punched geotextile on top of the soft soil at the base of excavation, about two metres in depth. A graded mix of crushed brick aggregates and coarse sand was placed on the top of the geotextile. The aggregates were vibro compacted, in layers, to desired density, by using twin drum vibratory rollers. A layer of locally available fine river sand was placed by vibro compaction on top of the coarse aggregate layer. The total thickness of the mattress, comprising the geotextile, coarse and fine aggregates, varied between 1.8 and 2 m depending on the loading condition. Continuous footings were designed for masonry as well as framed structures, which were cast on the mattress. A brief description of the functions of the mattress is presented. The deflection and distortion behaviour of the continuous footings is also presented as comparative studies of cases with and without granular mattresses.

INTRODUCTION

Khulna City, situated in the south east of Bangladesh, is generally underlain by soft Recent swamp deposits containing layers of decomposed mangrove vegetation. The soft soil areas of Bangladesh showing the location of Khulna area are shown in Figure 1. Non existence of bearing layer at moderate depths poses problems for construction of buildings between 4 and 6 stories, which are the more desired types in this area. Buildings above 6 stories are founded on piles whose length often exceed 30 m.

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